Descrição • Usos / Função

Discovered by Berzelius in 1817, who found it associated with tellurium, named for the earth. Selenium is found in a few rare minerals,such as crooksite and clausthalite. In years past it has been obtained from flue dusts remaining from processing copper sulfide ores, but the anode mudsfrom electrolytic copper refineries now provide the source of most of the world’s selenium. Selenium is recovered by roasting the muds with soda orsulfuric acid, or by smelting them with soda and niter. Selenium exists in several allotropic forms. Three are generally recognized, but as many as sixhave been claimed. Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, inpowder form, or black, in vitreous form. Crystalline monoclinic selenium is a deep red; crystalline hexagonal selenium, the most stable variety, is ametallic gray. Natural selenium contains six stable isotopes. Twenty four other isotopes and isomers have been characterized. The element is a memberof the sulfur family and resembles sulfur both in its various forms and in its compounds. Selenium exhibits both photovoltaic action, where light isconverted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These propertiesmake selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to converta.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses inelectronic and solid-state applications. It is used in Xerography for reproducing and copying documents, letters, etc. It is used by the glass industryto decolorize glass and to make ruby-colored glasses and enamels. It is also used as a photographic toner, and as an additive to stainless steel. Elementalselenium has been said to be practically nontoxic and is considered to be an essential trace element; however, hydrogen selenide and other seleniumcompounds are extremely toxic, and resemble arsenic in their physiological reactions. Hydrogen selenide in a concentration of 1.5 ppm is intolerableto man. Selenium occurs in some soils in amounts sufficient to produce serious effects on animals feeding on plants, such as locoweed, grown in suchsoils. Selenium is priced at about $150/kg. It is also available in high-purity form at a cost of about $250/kg (99.999%). 1

•&nbsp"toxicity has long been known" 2
•&nbsp"is known to be physiologically important (it is involved in the activity of vitamin E and certain enzymes)" 3
•&nbsp"deficiency has been shown to be connected to the occurrence of congestive heart failure" 4
•&nbsp"considered to be an antioxidant nutrient because it is present in enzymes that help protect against toxic species of oxygen and free radicals. Selenium deficiency is a major public health problem in certain parts of China, where it increases the risk of heart disease, bone and joint disorders, and liver cancer. Selenium is currently under intensive investigation as a possible protector against cancer." 5
•&nbsp"used in photocopy machines and in solar cells." 6

Bem vinda!

Chemistry-Reference.com provides you with capsules on many topics in chemistry. Additionally, there are reference tables and profiles of every element and thousands of compounds. Use the interactive menu bar located above to get started. Many online periodic tables only provide basic information for a particular element. Here, a full profile of the element's physical properties, chemical behavior, isotopes, and electron structure are provided. For further reading and reference, list of sources can be found at the conclusion of each page.